High pressure sodium vapor lamps have been in use for years and have been widely used for street, roadway and various other outdoor applications. These lamps are generally operated by a conventional ballast comprising windings on an iron core supplied from a 60 Hertz current power supply. The power supply and ballast are designed to limit the current through the lamp and provide a power input which does not exceed the lamp voltage rating. Generally, ballasts contain a special circuit for generating a high voltage low energy pulse to ignite the lamp by causing an arc to jump the lamp electrodes. The magnitude of this high voltage pulse is dictated by the lamp specifications. For example, a 400 watt lamp typically will call for a 1.mu. sec long pulse of 2250 volts amplitude (minimum) which is applied across the lamp electrodes at a repetition rate of least 50 times per second. This high voltage pulse breaks down the amalgam of sodium and mercury in a xenon gas atmosphere into charged particles. The voltage strikes across the electrodes of the lamp to ignite the lamp. The generation of light from the lamp is caused by a discharge through the sodium vapor in the lamp and is due almost exclusively to the excitation of the sodium atom. Once the lamp starts, the high voltage pulsing circuit is automatically disabled. The prolonged or steady operation of the lamp is then sustained by a prescribed current and voltage providing power input at the lamp's rating. In conventional a.c. operation, as the sodium is vaporized by the heat generated within the lamp, the light turns first to a monochromatic yellow and then gradually to white having a golden or orange cast. Full warm up of the lamp takes about one minute. Lamps operated in the above described manner are extremely useful for outdoor lighting. However, the light cast by such lamps is not suitable for indoor use where premium (white) light or color discrimination are requirements. Various prior art systems have been designed with attempts to obtain the above premium lighting from sodium vapor lamps. These systems and attempts, for the most part, use pulse operation to achieve higher lamp color temperature and improved color rendition while maintaining the average energy input into the lamp at a rated level. U.S. Pat. No. 4,137,484, Osteen (assigned to the assignee of the present invention) discloses one such prior art pulse operated system (inter alia), which patent is incorporated herein by reference for a teaching of the basic principles of operation of those sodium vapor lamps of the type contemplated by the present invention, and in particular to prior art methods of ballasting such lamps using pulsed operation. The metal of a conventional high pressure sodium lamp of the type contemplated by the present invention, and as disclosed in the system of U.S. Pat. No. 4,137,484, contains sodium and usually mercury. The mercury radiation produced by discharge through the sodium is insignificant, however, some radiation from the mercury does appear. In such a system, it is disclosed that, in the time interval during and immediately following the application of a pulse having a rapid rise to the lamp, the higher electronic states of sodium are excited to substantial emission, and in lamps containing mercury, radiation from mercury also appears, but it is insignificant. During pulse operation of the lamp, emission from several sodium lines and a continuum in the blue-green portion of the light spectrum becomes substantially more intense. In addition, the normal light in the yellow-red portion of the spectrum, which is due to self-reversal and broadening of the sodium D lines, is partially suppressed. As a result, an increase in color temperature and an improvement in color rendition index takes place.
The system disclosed in the above patent does not employ any "keep alive" current which can be defined as a constant current over which the pulsed current to the lamp is superimposed, such keep-alive current being intended to provide some current during pulse "off" conditions. Nor does the present invention. A keep-alive current can have a detrimental effect on the highly excited sodium radiation (and mercury radiation if mercury is present) on which the color improvement depends.
The above patent, while it discloses a system and method which provides improved color temperature and enhanced color rendering by powering the lamp with a pulsed current waveform, has certain disadvantages. One of these disadvantages is in a reduction in efficacy over conventional a.c. operation. In the pulsed current waveform system, pulses are used having repetition rates above 500 Hz up to about 2000 Hz and duty cycles from 10% to 35%. Using these pulse rates enables the lamp color temperature to be increased in excess of 400 degrees Kelvin(400 K.). That is, from the normal temperature of about 2050 K. up to about 2500 K. with about a 20% reduction in efficacy. It is also disclosed that color temperature can be raised considerably beyond 2500K but, at the sacrifice of further reduction in efficacy. Thus, it can be seen that a need exists to provide a system and method of pulse operating a sodium lamp which at least improves any such reduction and with increases in color temperature above the norm.
The above patent also recognizes that there can be wall darkening of the lamp which is promoted by the use of both unidirectional and bidirectional pulsing. It is known that wall darkening can be caused by a loss of sodium over time. This loss can be caused by operating the lamp well above its design rating whereby a higher vapor pressure is achieved. However, we have also discovered that wall darkening can also be caused by arc instability within the lamp. Because of this characteristic, we found that if a pulsed current waveform contains harmonics which coincide with the acoustic resonant nodes of the sodium filled arc tube, and if they are of sufficient magnitude during arc strike, an instability will occur in the arc. That is, the arc instead of being confined solely between the electrodes of the lamp, gets distorted or bends to strike the wall of the lamp. This striking can cause overheating, thus raising internal pressures which cause sodium to escape from the lamp and therefore reduce the expected life of the lamp.
Thus it can be seen a further need exists for a system, circuit and method of pulse operating a sodium vapor lamp which prevents the above arc instability problems and which provides a lamp having long life, "high" lumen maintenance and overall lower costs at least partly attributable to the elimination of any keep alive current circuitry.